A continuing objective in the design of gun propellants is to provide a gun propellant which is energetic when deliberately ignited, but which exhibits high resistance to accidental ignition from heat, flame, impact, friction, shock, and chemical action. Propellants possessing such resistance to accidental ignition are known as "low vulnerability ammunition" (LOVA) gun propellants.
Conventional LOVA gun propellants comprise a cured elastomeric binder, throughout which are dispersed particulates of high-energy material, particularly oxidizers. The elastomeric binder is generally a cured elastomer, i.e. thermoset, formed, for example, by the urethane reaction of a multi-functional prepolymer with a multifunctional isocyanate. Examples of such conventional propellants are described, for example, in U.S. Pat. Nos. 4,263,070 and 4,456,493, the disclosures of which are incorporated herein by reference. Generally, LOVA propellant grains are formed by extrusion at elevated temperatures whereat substantial curing takes place. Because the grains cure to some extent as they are being formed, control of extrusion conditions is difficult. If cured, LOVA propellant is unusable, it cannot be recycled, and burning the propellant is generally the only suitable disposal method.
Another type of LOVA propellant has a binder of cellulose acetate or a cellulose acetate derivative. An example of this type of propellant is described in U.S. Pat. No. 4,570,540, the disclosure of which is incorporated herein by reference. These types of LOVA propellants are solvent processed, a process which entails relatively long processing times and a large number of steps. Also, the use of solvent creates environmental problems.
Yet another type of LOVA propellant is formed from a thermoplastic elastomer and particulates of high energy oxidizers, e.g. cyclotetramethylene-tetra-nitramine (HMX), or cyclotrimethylenetrinitramine (RDX). Examples of this type of propellant are described in U.S. Pat. Nos. 4,919,737 and 4,976,794, the disclosures of which are incorporated herein by reference. Such propellants typically comprise between about 60 and about 85 wt. percent of high energy oxidizer particulates and between about 15 to about 40 wt. percent of a binder system which is plasticized or unplasticized block copolymer having at least one crystalline block and at least amorphous block, giving the block copolymer, thermoplastic elastomeric characteristics.
Thermoplastic elastomers have been previously used in propellants for rocket motors or the like, for example, as described in U.S. Pat. No. 4,361,526, the disclosure of which is incorporated herein by reference. Gun propellants, however, are considered to be a different art than rocket motor propellants. Rocket motor propellants typically contain a particulate metal fuel, e.g., particulate aluminum. Gun propellants, on the other hand, should be substantially free of any metal, and for that matter, should be generally free of any material which leaves a solid residue in the barrel of the gun upon burning. Gun propellants should also be substantially free of chlorine, which degrades the gun barrel.
Furthermore, rocket motor grains are typically formed in a different manner. Gun propellant grains typically take their shape from the extrusion process and must be sufficiently solid when leaving the extruder to retain their extruded shape. Material for rocket motor propellants may be extruded, but generally large rocket motors assume their shape from a mold, e.g., the rocket motor case; thus, after leaving an extruder or mixer, a propellant composition for a rocket motor should be free-flowing or at least moldable so as to be able to assume the shape of the large mold.
The conventional TPE-LOVA propellants have suffered from undesirably low mechanical properties. Prior efforts to utilize TPE-based propellants have been frustrated in part by an inability to prepare a material which exhibits low viscosity at mixing temperatures while having sufficient structural rigidity during potential propellant storage temperatures. Efforts to solve these and other problems have included decreasing the amount of plasticizer in the propellant. Although reducing the amount of plasticizer may impart certain useful high temperature mechanical properties, unfortunately it concurrently and undesirably reduces the propellants impetus by a significant amount. This is a significant drawback inasmuch as a combination of both high impetus coupled with desirable mechanical properties are critical to obtaining a successful LOVA propellant.